Flask



FIG. 1 is a front view of the present design;

FIG. 2 is a side thereof;

FIG. 3 is a top perspective view thereof;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof.

The broken lines in FIG. 5 represent a portion of the structure that forms no part of the claim. 

The ornamental design for a flask, as shown and described. 